This study examined stool and urine energy losses in obese versus lean individuals on two standard (2400 kcal/d and 3400 kcal/d each given over 3 days) diets In collaboration with Dr. Jeffrey Gordons lab at Washington University in St. Louis, samples of stool will be sent for determination of the fecal bacterial flora phylotypes to examine the association between stool energy loss and enteric flora populations. Preliminary results from an initial 14 lean and 9 obese individuals failed to show any difference in stool or urine energy loss. The range of calories stool was large, varying from 2 to 9 % in both groups. Lean individuals absorbed more (had less stool energy loss) on the higher calorie diet compared to obese individuals. Percent changes in the major gut phylotypes, Firimutes and Bacteroidetes, were associated with changes in nutrient load. Percent of Firmicutes increased with nutrient load while Bacteroidetes decreased. In addition, in lean individuals, these changes in the major phylotypes were associated with stool energy loss, such that a 20% increase or decrease in Firmcutes/Bacteroidetes was associated with approximately 150 kcal/day difference in stool energy loss. Based in this preliminary work, an amendment to this study is planned in which stool energy loss and changes in gut microbiota will be measured with over and underfeeding (150 and 50% based on calculated weight maintaining calories). This study will also randomize volunteers to antibiotics which will selectively change the gut bacterial population so we can establish a causal association between changes in gut microbiota and nutrient harvest (as measured by stool energy loss).